1. Field of the Invention
This invention relates to an edge dam position control device and method of controlling the upper and lower positions of an edge dam, which is installed on both edge faces of twin rolls in a twin roll strip casting process that produces a strip(hot coil) directly in melt, without having a process of producing slab. More particularly, this invention helps minimize the force applied to the edge dam during the casting, minimize the wear and tear of the edge dam and improve the quality of both end faces of a strip, by calculating the height of the solidification point using the roll reduction ratio and the roll reduction force, and adjusting the height of the edge dam in the casting to correspond to the height of the solidification point.
2. Discussion of Related Art
Referring to FIGS. 1 and 2, the strip casting method in a conventional twin roll strip casting device will be discussed. First, melt 207 is received within a ladle 200 and flows to a tundish 210 through a nozzle 205. Then, the melt 207 goes down to the space between a pair of rolls 220 and edge dams 230 that are mounted on the end faces of the pair of rolls 220. Next, the melt 207 is solidified on the surface of the rolls 220 rotating in opposite directions. The solidified shells 227 meet each other at the solidification point, which is generally above the roll nip point, which is a roll kissing point. Hence, the solidified shell is hot rolled at this stage.
Then, the cast strip 240 through the roll nip point passes through a cooling process and is coiled by a coiling system(not shown). In the above process, the thickness of the strip 240 is adjusted in accordance with the control of the interval between the rolls 220, and an adequate reduction of the solidified shell 227 is performed by means of a rolling force control unit 235, which is comprised of the roll assembly, hydraulic systems and control system. In this case, the rolling force of the rolls 220 can be measured by the load cell, which is connected to the cylinder rod 237b supporting a roll chock 220a.
Hence, in the twin roll strip casting process that directly casts the strip 240 with about 10 mm or less thickness from the melt 207, what is important is that the melt 207 should be properly infused into the space between the water cooled twin rolls 220 through the nozzle 225 from the tundish 210, so that the strip 240 can be produced with the desired thickness.
As shown in FIG. 1, in the conventional edge dam position control method, the bottom of the edge dams 230 is located on a roll nip point 222. Japanese Laid Open Application No. 4-46656 discloses a structure to support the edge dam 230 with a predetermined force using a hydraulic device against both end faces of the rolls 220.
In the above prior art, however, it is noted that upon a casting operation, the edge dam 230 is moved backward in a horizontal direction(in a vertical direction to the sheet of FIG. 1) due to the rolling force of the rolls 220 or the formation of skull(not shown) on a pool of the melt 207. Skull is a solidified slab which is formed by local solidification at the circumference of a nozzle on the melt pool surface or at the joint between an edge dam and molten metal. In this case, the force applied to the edge dam 230 is maintained at a constant state by means of the hydraulic device. In general, a main object of the edge dam is to prevent the leakage of the melt 207 from the both sides of the rolls 220. But in this case, the main object of edge dam cannot be achieved. Thus, good quality of the edge of the strip 240 may not be obtained.
In detail, when the edge dam 230 is moved backward due to the rolling force of the rolls 220 or the formation of the skull on the pool of melt 207, the melt 207 has a leakage through a crevice. This results in an irregular formation of edge flash on both edge faces of the strip 240, deteriorating the quality of the strip 240. When the solidified shell is inserted between the edge dam 230 and the rolls 220, the edge dam 230 and the twin rolls 220 are extremely damaged. Moreover, when the edge dam 230 is supported by a constant force against the rolls 220, a serious problem occurs because the edge dam 230 or the rolls 220 are extremely damaged on the side.